In the investigation of Loch Ness monster, people finally gave the result that dead wood floated on the water. Water ghosts, some people used to mistake sloths for water ghosts, just because sloths look strange. The Japanese river boy just lives in legend.
It is conceivable that most of these sensational water monster legends are people's fear of the unknown, just like people's fear of the night, so they fantasize about ghosts, monsters, zombies and vampires in the night. Some people are afraid of deep water because there may be unexpected things in the cold and dark deep water, such as corpses, sharks, piranhas, unknown creatures and so on.
And people's fantasies about whether there are terrible creatures in deep water are gradually enlarged in people's mouths. For example, "I seem to see something moving in the water", "He saw something moving in the water, so it can't be a monster" and "Someone in the water here has seen a monster, which is terrible" ...
However, I think the so-called water monster may exist, but people have different definitions of it. It is often defined as a monster in the water, terrible, eating people and so on. I think monsters should be defined as unknown creatures in the water, such as the Loch Ness monster. If he is a creature left over from the dinosaur era, it is not impossible, but we can't find him to confirm its existence. Even if he does exist, it's just another species. We can think that he is a crocodile. Even if this species is very dangerous, we can treat it like a tiger and a lion. They have no influence on us at all. If it could really do us any harm, we would have done it long ago.